This invention relates generally to the field of apparatus for bending tubes. More particularly, the invention is a gripper and mandrel assembly for use in a tube bender. The assembly assures a firm grip on the tube and controls wrinkling of the tube wall and interior cross sectional area during a bending operation. The invention is particularly suited for use in a stretch-type tube bender. The invention was conceived for use in a tube bender that makes return bends for air-to-refrigerant heat exchangers in air conditioning and refrigeration systems but could be used for bending tubes for other purposes as well.
A typical air conditioning or refrigeration air-to-refrigerant heat exchanger is a system of tubing through which refrigerant flows. Air flows over the tube exterior and heat transfers through the tube wall from the fluid having the higher temperature to the other fluid. Such a heat exchanger is typically manufactured by passing a number of U-shaped tubes, called "hairpins," through tubesheets and, in most cases, a stack of plate fins. The tubesheets maintain the tubes in a predetermined relationship with each other and give the heat exchanger increased physical strength. The plate fins increase the external surface area of the tube and thus improve heat transfer performance. The plate fins also give increased strength and rigidity to the assembly. After "lacing" the tubes through the tubesheets and fin stacks, two ends of adjacent hairpins are joined with a short U-shaped tube, called a "return bend," to form a single flow path between the two hairpins. Not every hairpin end will be joined with a return bend but the typical heat exchanger will have a large number of return bends. The air conditioning and refrigeration industry makes and uses millions of return bends yearly.
In manufacturing return bends, it is necessary that there be no "peglegged" return bends, that is, the legs of the return bend must be exactly the same length. It also necessary that the exterior wall of the return bend adjacent each of its ends be smooth and undamaged. These requirements must be met so that the joint between a return bend and a hairpin will fit up correctly and then can be brazed, welded or soldered to become fluid tight.
In several types of tube benders, including the stretch type, one or both of the ends of the workpiece being bent must be securely grasped in order to effect the bend. Obtaining a good grip on an end often results in damage to the workpiece in the area at the end being gripped. Sometimes the damage is sufficient to make trimming off the damaged end necessary.
A prior art manufacturing process for making return bends that is in current widespread use uses a rotary draw bender. This process produces return bends with legs that are longer than required. After bending, the excess length in the return bend legs is cut off A saw-like cutting tool must be used so that this process produces saw chips and results in a great deal of scrap, both from the excess length of tube cut off as well as the chips produced in the cutting process. After cutting, chips from the saw cutting must be removed from the finished return bends, a process that involves the use of cleaning chemicals. In a large scale return bend manufacturing operation, the amount of scrap and chemical waste produced can be considerable.
In order to insure that proper fluid flow rates are attained in a tube type heat exchanger, it is necessary that the inside cross sectional area of all portions the completed fluid flow path be at least some minimum value. A tube bending operation frequently wrinkles that portion of the tube wall that is on the inside of the bend as well and flattens that portion of the tube wall that is on the outside of the bend. Both wrinkling and flattening can result in reduction in the cross sectional area of the bent tube, a reduction that may result in a cross sectional area that is below the minimum acceptable value.
What is needed is a tube bender that can make return bends with precisely controlled dimensions while producing a minimum of scrap.